Complex microelectronic devices such as modem semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor die may require hundreds or thousands of connections to external devices.
The size of the resulting chip structure is a major concern. Many times the size of each such structure influences the size of the overall electronic product. Moreover, the size of each assembly to some extent controls the internal impedance of the microelectronic device and also the required distance between each chip and other chips, or between each chip or other elements of the circuit Delays in transmission of electrical signals between chips are directly related to both the internal impedance's and these distances. These delays limit the speed of operation of the device. For example, in a computer where a central processing unit operates cyclically, signals must be interchanged between the central processing unit chip and other chips during each cycle. The transmission delays inherent in such interchanges often limit the clock rate of the central processing chip. Thus, more compact interconnection assemblies, with smaller distances between chips and smaller signal transmission delays can permit faster operation of the central processing chip.
Further, compensation for the substantial stress caused by thermal cycling as temperatures within the device change during operation are of major concern. The electrical power dissipated within the chip heats the chip and its supporting substrate, such as printed wiring boards ("PWB"), so that the temperature of the chip and PWB rises each time the device is turned on and falls each time the device is turned off. As the chip and the PWB ordinarily are formed from different materials having different coefficients of thermal expansion, the chip and PWB expand and contract by different amounts ("CTE mismatch"). This causes the electrical contacts on the chip to move relative to the electrical contact pads on the substrate as the temperature of the chip and PWB changes. This relative movement deforms the electrical interconnections between the chip and PWB and places them under mechanical stress. These stresses are applied repeatedly with repeated operation of the device, and can cause fatigue of the electrical interconnections. Thermal cycling stresses may occur even where the chip and PWB are formed from like materials having similar coefficients of thermal expansion, because the temperature of the chip may increase more rapidly than the temperature of the PWB when power is first applied to the chip.
The cost of the chip and PWB assembly is also a major concern. All these concerns, taken together, present a formidable engineering challenge. There are many different ways used to attach die to PWBs all of which can be broken down into two groups: unpackaged die and packaged die.
An example of unpackaged die includes direct chip attachment to the PWB using many different kinds of electrical/mechanical means. One embodiment of a direct chip attach method is commonly referred to as flip chip or C4. In flip-chip bonding, contacts on the front surface of the die are provided with bumps of solder. The PWB has contact pads arranged in an array corresponding to the array of contacts on the die. The die, with the solder bumps, is inverted so that its front surface faces toward the top surface of the PWB, with each contact and solder bump on the chip being positioned on the appropriate contact pad of the PWB. The assembly is then heated so as to liquefy the solder and bond each contact on the die to the confronting contact pad of the PWB. Because the flip-chip arrangement does not require leads arranged in a fan-out pattern, it provides a compact assembly. The area of the substrate occupied by the contact pads is approximately the same size as the die itself. Moreover, the flip-chip bonding approach is not limited to contacts on the periphery of the die. Rather, the contacts on the die may be arranged in a so-called "area array" covering substantially the entire front face of the die. Flip-chip bonding therefore is well suited to use with chips having large numbers of I/O contacts. However, assemblies made by flip-chip bonding are quite susceptible to thermal stresses. The solder interconnections are relatively inflexible, and may be subjected to very high stress upon differential expansion of the chip and substrate. These difficulties are particularly pronounced with relatively large chips. For this reason, flip chip dies are underfilled with a curable liquid epoxy after attachment to a PWB in an attempt to compensate for the CTE mismatch. This underfill process is very expensive. Further, flip chip requires expensive routing layers to be placed on the face of the die. Further still, it is difficult to test and operate or "burn-in" chips having an area array of contacts before attaching the chip to the substrate. Moreover, it is virtually impossible to standardize a flip chip type die. In packaging, it is crucial that standards be reached so that the end user can meet its semiconductor chip needs from multiple companies--this allows for a more price competitive market place for the end user which is insensitive to the production and defect variations of any one chip manufacturer. Other examples of unpackaged, non-standardizable solutions include U.S. Pat. Nos. 5,476,211 and 5,495,667.
Examples of packaged die include ball grid array ("BGA") and chip scale packaged ("CSP") semiconductor chips which are connected to external circuitry through the package interface. The external terminals on these packages are generally either disposed in regular grid-like patterns, substantially covering the face surface of the chip (commonly referred to as an "area array") or in elongated rows extending parallel to and adjacent each edge of the chip front surface. BGA and CSP technology generally refers to a large range of semiconductor packages which typically uses an interconnection process such as wirebonding, beam lead, tape automated bonding ("TAB") or the like as an intermediate connection step to interconnect the chip contacts to the exposed package terminals. This results in a standardizable, testable device prior to mechanical attachment to the bond pads on a PWB. The BGA or CSP packaged chips are then typically interconnected with their supporting substrates using standard tin-lead solder connections. In most such packaged devices, the mechanical stress/strain due to thermal cycling (the heating and cooling cycles of the chip during operation) is almost completely placed on the solder connections between the chip and the substrate. However, as described above, solder was never intended to undergo such forces and many time encounters significant elastic solder deformation causing the solder to fatigue brought on by the thermal cycling. As the solderballs get smaller in diameter, thermal cycling has an even more profound fatiguing affect on the solder.
As the features of semiconductor chip packages continue to be reduced in size, as in the case of CSPs, the number of chips packed into a given area will be greater and thus the heat dissipated by the each of these chips will have a greater effect on the thermal mismatch problem. Further, the solder cracking problem is exacerbated when more than one semiconductor chip is mounted in a package, such as in a multichip module. As more chips are packaged together, more heat will be dissipated by each package which, in turn, means the interconnections between a package and its supporting substrate will encounter greater mechanical stress due to thermal cycling. Further, as more chips are integrated into multichip modules, each package requires additional interconnections thereby increasing the overall rigidity of the connection between the module and its supporting substrate.
Several patented inventions developed and assigned to the present assignee deal effectively with this BGA/CSP thermal mismatch problem, but in a specifically different way than the present invention. The patent numbers of some of these commonly owned patents are U.S. Pat. Nos. 5,148,265, 5,148,266, 5,258,330, 5,346,861, 5,347,159, 5,414,298, 5,455,390, and 5,518,964 to name a few.
Despite the positive results of the aforementioned commonly owned inventions, still further improvements would be desirable.